In recent years, a display device has been developed from a display device with using a cathode-ray tube to a flat-type display device called a flat panel display (FPD) such as a liquid crystal display and a plasma display. In the FPD, for a switching element for display switching by liquid crystal, a thin-film transistor is adopted. For example, for a switching element of the liquid crystal display, a thin-film transistor in which amorphous silicon or polycrystalline silicon is used for a channel layer is adopted. For such a FPD, it is required to add new functions such as further increase of the area and flexibility, and it is required to provide high performance as an imaging element device as well as handle a process capable of manufacturing the large-area FPD and handle to a flexible substrate.
Also, recently, in order to achieve a further larger area or more flexibility, an organic EL display with using organic electro luminescence (EL) has been developed. Also in the organic EL display, for the switching element, a thin-film transistor is adopted. However, the organic EL display is a self light emitting device which directly obtains light emission by driving an organic semiconductor layer, and therefore, characteristics as a current-driving device are required for the thin-film transistor as different from the liquid crystal display.
From such a background, in order to improve transistor characteristics of the thin-film transistor for the display device, in recent years, a study has been made, in which an oxide semiconductor whose band gap is large as about 3 eV, which is transparent, and which can be deposited at a low temperature is adopted for a channel layer of the thin-film transistor. The adoption of the oxide semiconductor for not only the display device but also a thin-film memory or individual discrimination (Radio Frequency Identification: RFID) by radio waves or others has been expected.
Although zinc oxide or tin oxide is generally used as the oxide semiconductor forming the channel layer of the thin-film transistor, a problem of easy threshold potential shift arises for the thin-film transistor with using the materials. Accordingly, as an oxide semiconductor capable of suppressing the threshold potential shift of the thin-film transistor, indium gallium zinc oxide (IGZO) has been proposed.
For example, Japanese Patent Application Laid-Open Publication No. 2006-165532 (Patent Document 1) discloses a semiconductor device which includes a P-type region and an N-type region and in which amorphous oxide whose electronic carrier concentration is lower than 1018/cm3 is used for the N-type region, and discloses that the amorphous oxide is oxide containing indium (In), gallium (Ga), and zinc (Zn).
Also, Japanese Patent Application Laid-Open Publication No. 2006-173580 (Patent Document 2) discloses a field effect transistor which includes an active layer made of amorphous oxide whose electronic carrier concentration is lower than 1018/cm3 or amorphous oxide having a tendency of increase in electronic mobility together with the increase in the electronic carrier concentration, and the field effect transistor in which at least one of a source electrode, drain electrode, and a gate electrode has transparency to visible-range light, and disclose that the amorphous oxide is oxide containing In, Zn, and Ga.
It is confirmed that the thin-film transistor in which the IGZO is adopted for the channel layer has a better subthreshold slope value than that of the thin-film transistor in which the polycrystalline silicon is adopted for the channel layer. Also, the adoption of the IGZO for not only the display device but also other devices requiring extremely-low voltage operation or extremely-low power consumption has been expected. However, on the other hand, the IGZO contains In or Ga of a significant amount in which exhaustion of its resource or uneven distribution of the resource is concerned, and therefore, the IGZO would be disadvantageous for industrial use in future.
Accordingly, the inventors of the present application have studied on zinc tin oxide (ZTO) as an oxide semiconductor without In. As a result, in the thin-film transistor in which the ZTO is adopted for the channel layer, transistor characteristics equivalent to that of the thin-film transistor in which the IGZO is adopted for the channel layer have could been obtained. Also, it is assumed that, in future, a large market would be established also for the oxide semiconductor target used in depositing the oxide semiconductor by a sputtering method or a deposition method. However, at that time, from a point of view in a cost or securing of the resource, it is considered that a material as the ZTO without rare metal is more advantageous than a material with rare metal such as In and Ga.